In the midst of general trends towards finer structuring of semiconductor devices, development activities have been remarkable in the field of microcomputers having capacitors of high dielectric constant materials which is effective to reduce unnecessary radiation, or electro-magnetic interference, and in the field of non-volatile RAM having built-in ferroelectric capacitors which facilitates low-voltage operation and high read/write speed. the ferroelectric materials are made mainly of metal oxide, and contain substantial amounts of very reactive oxygen. In forming a capacitor with such dielectric film, material for its electrodes must be least reactive; precious metals such as Pt, palladium, etc. must be used.
FIG. 9 shows a typical structure of a capacitor in a semiconductor device having a built-in dielectric capacitor. In FIG. 9, 1a denotes a top electrode, 2a a capacitance insulation film, 3a a bottom electrode, 4 an underlayer, 7 a contact hole, 8 an inner insulation layer (oxide layer) and 9 wiring.
In the above mentioned structure, the bottom electrode 3a is made larger than the top electrode 1a, in order to provide contact individually at two points, viz. top electrode 1a and bottom electrode 3a, with the upper wiring 9.
The etching process to form the capacitor consists of two processes. One is etching of a Pt top electrode layer and dielectric film, and the other is etching of a Pt bottom electrode layer 3; wherein, etching on the Pt top electrode layer and dielectric film must be conducted at a speed different from that on the Pt bottom electrode.
In the prior art, Pt etching has been conducted by means of wet etch with aqua regia, ion milling with Ar gas, or by other means. However, the nature of both wet etching and ion milling is isotropic etching, which means the grade of precision is not high enough for the fine pattern processing. Especially, ion milling bears with it a drawback that there is not much difference in etching speed between Pt and underlayer material.
In order to overcome the above mentioned drawback, active research and development efforts have been made in Pt fine pattern processing by means of dry etching; where, as etching gas, use of Cl.sub.2 and HBr are reported (Extended Abstracts, Autumn Meeting 1991. the Japan Society of Applied Physics, 9p-ZF-17, p. 516; Extended Abstracts, Spring Meeting 1993, The Japan Society of Applied Physics, 30a-ZE-3, p577, and others). What follows is an explanation of the dependance of Pt etching speed to high frequency (RF) electric power when Cl.sub.2 gas is used as etching gas.
FIG. 10 shows the dependence of Pt etching speed, and of Pt/resist etching speed ratio to RF power, with Cl.sub.2 gas as etching gas. The horizontal axis denotes RF power, the left vertical axis Pt etching speed, and the right vertical axis Pt/resist etching speed ratio.
The facility used for the etching is a magnetron reactive ion etching (RIE) mode dry etcher.
Etching conditions are: Cl.sub.2 gas flow 20 SCCM, gas pressure 1 Pa. Wafer temperature during etching is below 20.degree. C. because its back surface is cooled with He.
FIG. 10 exhibits a trend that Pt etching speed is increased from 10 nm/min. to 100 nm/min. when RF power is raised from 200W to 600W.
Pt/resist etching speed ratio, when RF power is raised from 200W to 600W, also shows an upward trend from 0.2 to 0.3; however, the etching speed ratio remains very low. The above quoted papers also report similar results in the dependence of Pt etching speed to RF power when Cl.sub.2 gas is used.
In addition, the above cited papers describe the case where HBr gas was used as etching gas; in which case, according to the papers, Pt etching speed was reported to be 120 nm/min.
In such conventional cases where single substance gas such as Cl.sub.2 or HBr is used as etching gas, Pt etching speed is very low; which means it takes a very long time to etch Pt of a required thickness (several hundreds nm). Therefore, a drawback of this method is deteriorated throughput of production facilities.
Furthermore, the prolonged etching time may cause a problem of etched resist layer during etching time. If the resist layer is made thicker, it affects the grade of definition, making formation of a fine pattern very difficult.
A possibility for increasing Pt/resist etching speed ratio is to add a gas containing carbon component and to conduct etching by means of plasma polymerization while piling up the carbon layer. This procedure, however, causes piling up of the carbon layer within the reaction chamber, which becomes a source of particle generation. This necessitates frequent maintenance servicing of facilities, making operation efficiency poor.